For subsurface geotechnical exploration, it is customary to drive a 2 in. O.D. split barrel sampler into the soil to obtain a sample and record the number of blows required to drive the sampler a specified distance to determine the penetration resistance of the soil. The penetration resistance has been correlated to quantitative measures of soil behavior. An ASTM Standard D-1586-67 for this procedure known as the "Standard Penetration Test" or "SPT" is available. This standard in part states "The assembly shall consist of a 140 lb. (63.5 kg) weight, a driving head, and a guide permitting a free fall of 30 in. (0.76 m)." In the description of the preferred embodiment and in the claims, the "weight" is referred to as the hammer to distinguish it from the mass of one or more of the components.
The most common method of driving the sampler is by lifting a 140 lb. weight with a cathead and rope and releasing the rope allowing the weight to fall and impact on an anvil connected to a drill rod column connected to a split barrel sampler. A cathead is a rotating drum, usually about 8 in. in diameter. A rope is wound around the cathead for one or two turns. One end of the rope is positioned over sheaves at the top of a drill mast and is then attached to a 140 lb. weight. The other end of the rope is held in the hand of the operator. As he pulls the rope, the rope tightens around the revolving cathead. Friction from the rope-drum contact assists in lifting the weight. The weight is raised 30 in. and the operator quickly releases the rope allowing the weight to fall and impact against the anvil. The speed of the operation is usually 40 to 60 blows per minute. A problem with the cathead and rope method is its lack of accuracy. If the operator does not release the rope properly, there may be some retardation of the falling weight resulting in a loss of kinetic energy at impact. Also, a careless operator may not release the rope at the correct height to deliver a 30 in. fall, resulting in over-stroking or under-stroking. Over-stroking results in more energy's being delivered and under-stroking results in less energy's being delivered. To overcome this difficulty, a device known as a trip monkey is sometimes used, particularly outside the United States. With this device the operator raises the weight and when it reaches a predetermined height of 30 in., a latch mechanism contacts a cam surface, allowing the weight to disengage, free fall for 30 in., and impact against an anvil connected to the drill rod column. The trip monkey is usually lifted with a cathead and rope or a hoist. The trip monkey offers an accurate stroke and a free fall of the weight without retardation. However, the trip monkey method is slower than using the cathead and rope method. The trip monkey usually operates at about 20 blows per minute. It is also bulky and more difficult to handle than the standard hammer. Both the cathead and rope drive weight and the trip monkey drive weight require a guide mechanism to insure axial impact with the anvil. The guide mechanism is both axially aligned with and rigidly connected to the drill rod column. The extra weight of the trip monkey mechanism may have an effect on the energy transmitted to the column.
The standard 140 lb. hammer is sometimes used with a wire rope and hoist. The hoist lifts the weight and the operator manually releases the hoist clutch to allow the weight to fall. The wire rope is attached to the weight and as the weight falls, the wire rope is pulled from the hoist drum. The problem with this method is that the inertial forces required to accelerate the hoist drum and frictional losses cause an often variable retardation of the falling hammer so that there is not a true free fall. The accuracy of the fall height with this method is also a problem.
Hydraulically-driven trip monkeys have been made that eliminate some of the objections of other methods. However, they have several disadvantages. They are bulky and hard to handle compared to conventional 140 lb. hammer systems. They are designed to rest on top of the drill rod column that connects to the split barrel sampler and, therefore, apply the weight of the device to the drill rods, affecting the energy transmitted to the rods. These types of devices provide an anvil for the weight to drive against. The lifting and tripping mechanism, however, is connected to the anvil; therefore, some of the impact energy is transmitted to the lifting and tripping parts causing energy losses and damage to the equipment.
The U.S. Army Corps of Engineers has also used a hydraulically driven hammer system, with two lifting lugs on a drive chain, but the use of two lugs limited the cycling speed, and the hammer operating mechanism was supported and guided by the drill rods in the hole, as in the device described above, and no provision was made for preventing the hammer's being operated when the device was not properly positioned.
The use of a driving system is not limited to the SPT. Other tests using different weights and drop heights are in use. Also, driving systems are used to drive drilling tools such as casing into the ground.
It is an object of this invention to provide a self-contained device for lifting a 140 lb. weight and allowing it to free fall 30 in. The device is automated so that the lifting and dropping of the weight is continuous.
Another object of this invention is to provide a device that allows the weight to fall against an anvil that is isolated from the device so that all of the energy from the falling weight is transmitted to the drill rod column.
Another object of this invention is to provide a device that is protected from the forces transmitted at impact, reducing damage to the device.
Another object of this invention is to provide a means for supporting much of the weight of the device so that the full weight is not resting on the rods and the sampler, affecting the accuracy of the test.
Another object of this invention is to provide a means for attaching the device to a drill rig so that it has an over-the-hole working position and an off-hole stored position out of the way of other drilling functions and can be easily moved from the stored position to the working position or from the working position to the stored position.
Another object of this invention is to provide a means to lift or lower the device, preferably hydraulically, so that the device can be vertically positioned.
Another object of this invention is to provide a lifting and tripping mechanism that will provide approximately the same rate of blows per minute as the cathead and rope method, in the range of 40 to 60 blows per minute.
Another object of this invention is to provide a guide system for the hydraulic hammer so that the hammer's vertical movement will be parallel to the feed mechanism of the drill rig and collinear to the axis of the drill rod column in the bore hole.
Another object of this invention is to provide a device that will easily allow the weight size to be changed and the stroke length to be changed.
Another object of this invention is to provide a safety feature so that if the anvil is not in place and the device is activated, the lifting mechanism will not lift the weight and allow it to fall, causing damage to the device.
Another object of this invention is to provide a safer means to drive the sampler, where all moving parts are enclosed.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.